\section{Background}\label{sec_background}

%http://www.webcredible.co.uk/user-friendly-resources/web-accessibility/uk-website-legal-requirements.shtml
The term CAPTCHA is an acronym which stands for ``Completely Automated Public Turing Test To Tell Computers and Humans Apart".  The Turing test refers to a scenario in which three people  are communicating. If a computer can take the place of one of the participants in the conversation without the other participant in the conversation, being able to recognise that there are conversing with a computer then that computer can said to be intelligent as no distinction can be made between it and a human it can be said that the computer has equal intelligence to a human.

The Turing test is referred to as being public. This is a requirement that it should prove highly challenging to write a computer program which would authenticate a computer as a human even though the method for authentication is known. This means that a CAPTCHA is a device which is used to prevent the automation by a computer of a task\footnote{\url{http://www.CAPTCHA.net}}\cite{turing}\cite{turingfifty}.

Typically a CAPTCHA is in the form of an image which displays distorted text. Tohis performs the Turing test by making it as difficult as possible for OCR (Optical Character Recognition) software to read.
\begin{figure}[h]
 \centering
 \includegraphics[width=80mm]{traditionalcaptcha.eps}
 \caption{Taditional CAPTCHA: the correct answer is "contribute of" }
\end{figure}

CAPTCHA are commonly used to prevent the abuse of online services by ensuring that a request for service is conducted by a human.  Programs known as "bots" are used to automate tasks such the proliferation of spam on online blogs and forums or to obtain free email address from providers such as Hotmail and Yahoo for use in sending spam via email.

CAPTCHAs are one of the most significant usability problems which face blind internet users\cite{tellinghumans}.  A blind person relies on the use of a screen reader to provide an interpretation of the contents, and visible actions, which occur on screen.  This is usually in the form of an audio description.  Screen readers are unable to read the contents of current versions of CAPTCHA as the screen reader represents exactly the kind of  automated process which a CAPTCHA is designed to defeat.  Thus the usability of a CAPTCHA designed for use by blind users is highly dependent on the functionality of the screen reader.  To provide a CAPTCHA  which is suited to the needs of blind people, its is vital to bear in mind level of comprehension the screen reader provides and accommodate it.


%\todo{Information on the weaknesses ofthe screen reader}

Providing inaccessible services through the internet can have serious consequences for the provider. Discrimination laws in many countries(UK, Australia\footnote{\url{http://www.austlii.edu.au/au/legis/cth/consol_act/dda1992264/}} and United States) state that people with disabilities should have the same access to services, which includes services provided through the internet. There have been numerous instances of legal action being taken against organisations and companies who have provided internet services which have not been accessible by people with disabilities, resulting in significant financial losses. For example a blind person successfully sued the Sydney Olympics organising committee for an inaccessible website\footnote{\url{http://www.oag.state.ny.us/media_center/2004/aug/aug19a_04.html}}. 


A CAPTCHA is a balance between the level of security and its usability.  The main focus of a CAPTCHA is to provide a test which is simple for a human but extremely challenging for a computer.  The usability issues which relate to the use of CAPTCHA are\cite{nasatlx}:
\begin{itemize}
\item Response time - How long does it take for a user to complete an authentication.  The user must deem that the time taken to complete the CAPTCHA is worth spending to access the protected resource.
\item How easy to use was the system - If the user deems the task to complex then they are unlikely to undertake it.
\item How mentally demanding was the system to use - If the user finds the to complex then this will result in users abandoning the authentication process.
\item How Frustrating was it to use the system -  Frequent errors and numerous stages will provide the user with the 
\end{itemize}

To perform analysis on the the level of security a CAPTCHA provides  the following criteria must be taken into consideration\cite{posh}:
\begin{itemize}
\item Un-solvability - There should not be any known efficient algorithm which will authenticate a computer as a human.
\item Non-malleability - The providing of a human solution to a single instance to a problem should not allow a computer to authenticate subsequent CAPTCHAs. 
\end{itemize} 


%http://www.w3.org/TR/turingtest/
\subsection{CAPTCHA methods}\label{subsect_alternate_methods}
CAPTCHAs are not limited to distorted text.  The Turing test, which is the foundation of a CAPTCHA, can be performed through numerous means.  CAPTCHAs can be classified into the following categories:

\subsubsection{Logic Puzzles}\label{subsubsec_logic_puz}
--- represent the Turing test through implied knowledge that a human would invariably have.  This method is most commonly provided through asking the user questions, for example ``how many figures do most humans have?" or asking how many items are in a photo.


\subsubsection{Sound Output}\label{subsubsec_soundouput}
--- requires the user interpret the played sound and to type in the the spoken text to represent the Turing test using a similar method to textual CAPTCHAs. The user, instead of entering the words displayed in the form of an image, reproduces words from a verbal audio clip


\subsubsection{Biometrics}\label{subsubsec_biometrics}
--- use of the unique identifiers which make us human.  These includes figure print, hand vein, retinal scan and DNA.  This solution is not ready for wide scale use due to the limited amount of hardware available to input biometric information on personal computers.  This solution may become prevalent amongst websites if the hardware becomes commonplace but there are still issues related to privacy and the ease of spoofing these devices in uncontrolled settings.

\subsubsection{Video}\label{subsubsec_videos}
---  the identification of the key themes or object within a video clip can also provide the Turing test. The creation of these CAPTCHA can be automated from the use of user-generated videos.  The verification of the user is performed through the macthing of the user's tags to that of the owner of the video\cite{youtubetagging}.  The problem with this method is that the video and associated tags are available on the internet, to defeat this all information pertaining to the identity of video must be removed.  This type of CAPTCHA can be very time consuming, which requires the user to watch the entire video clip before performing the authentication process.



\todo{re-read and cite paper}
\subsubsection{Images}\label{subsubsec_images}
--- the identification of images as the method for providing the Turing test.  Computers currently can not identify complex images. This method presents the problem of being able to automate the generation of the CAPTCHA. The labeling of an image can not easily be performed by a computer, so the creation of a CAPTCHA must therefore also be performed by a human, thus limiting the number of problems which can be created.  Services such as Amazon Mturk perform such automated tasks, through the outsourcing of these tasks to large numbers of humans, who provide solutions to problems which are beyond the capabilities of current computers. This technique is known as crowdsourcing which exploits the vast numbers of internet users to accomplish a task, in this case the labelling images.  Simple image CAPTCHAs which require the human distinction between objects of known categories could be provided. This is also different for a computer using a wide range of complex images, where few semantic tags or no details of the images are widely avialable\cite{asirra}



% second part of number four
\subsection{Previous work}\label{subsect_previouswork}
There have been many attempts to improve upon the traditional text based CAPTCHA system. Implementations provide very different methods for proving the Turing test. Although few have been widely used they provide insight into methods that could prove usable to blind people. Each CAPTCHA provides a different balance between Usability and Security.

 
\begin{figure}[h]
\label{fig_logicbased}
 \centering
 \includegraphics[width=80mm]{WWQuestion.eps}
 \caption{Logic based CAPTCHA}
\end{figure}
Figure \ref{fig_logicbased} is a logic based CAPTCHA which performs the turing test through requesting knowledge, that only a human should know.
 
\begin{figure}[h]
\label{fig_imagebased}
 \centering
 \includegraphics[width=80mm]{CMCaptchaWord.eps}
 \caption{Image based CAPTCHA}
\end{figure}
Figure \ref{fig_imagebased}The user presented with a series of images.  To need to select form a series of terms which relates to the images.

\begin{figure}[h]
\centering
 \includegraphics[width=80mm]{CMCaptchaTrace.eps}
 \caption{Picture based}\label{fig_trace}
\end{figure}
Figure \ref{fig_trace} is a CAPTCHA which requires the user to trace out the images which are displayed on the screen.  This provides a bot with two challenges, the first to identify which image to select and also to precisely identify which part of the image to highlight.


\begin{figure}[h]
 \centering
 \includegraphics[width=80mm]{CMCaptchaRecaptcha.eps}
 \caption{The Re-CAPATCHA System}\label{fig_recaptcha}
\end{figure}
Figure \ref{fig_recaptcha} is an example of traditional CAPTCHA combined with a sound version for usability

\subsection{Screen Readers}\label{subsection} 

It is necessary to check that a CAPTCHA is usable and accessible by all users.  This means we have to verify that it functions with the most common screen readers.  In a similar maner to checking that a webpage is constant across multiple browsers(Firefox, Internet Explorer and Safari), the CAPTCHA must function within the limitations and capabilities of the screen reader in order to be accessible.  The most commonly used screen readers are JAWS\footnote{\url{http://www.freedomscientific.com/products/fs/jaws-product-page.asp}}, HAL \footnote{\url{http://www.yourdolphin.com/productdetail.asp?id=5}} and Windows eyes\footnote{\url{http://atto.buffalo.edu/registered/ATBasics/Curriculum/Reading/rdrstable.pdf}}. Each provide their own shortcuts and levels of usability, to fully provide a highly usable CAPTCHA it must be compatible with the most commonly used screen readers.


%Number 6
\subsection{The problem for blind users}
Current methods of providing CAPTCHAs for the visually impaired use audio versions of CAPTCHA. This version is susceptible to automated use of speech recognition software, To counteract this, distorted sounds are added to defeat this type of attack.  This method is not ideal in noisy environments such as Coffee shops and offices where it can be challenging to distinguish between the environment's background noise, distortions which are used to defeat speech recognition software and the contents of the CAPTCHA. The poor level of usability provided by audio CAPTCHAs is compounded by the dialect in which the contents of the CAPTCHA are dictated. For example, a user in Glasgow may find it problematic to comprehend a CAPTCHA described in an American accent\cite{languageAccents}\cite{nonnative}.  Even  users who have perfect hearing can make numerous erroneous inputs and a frustrated user\cite{usabilitycaptcha}.  Hence in the attempt to provide accessibility to the blind, accessibility to the hard of hearing is reduced. \todo{Check this}

% How do you attack a CAPTCHA


\subsection{Aims of Project}\label{subsect_aim_proj}
The aim of this project is to provide an alternative and accessible CAPTCHA.  The CAPTCHA must have the following characteristics:
\begin{itemize}
\item Provide a CAPTCHA system which is inexpensive in time and financial resources to create and operate.
\item The CAPTCHA should successfully determine if the verification is being attempted by a human user or a computer.  
\item Should be designed around the needs of a blind user as well as being accessible to the sighted.
\item Should not be excessive in time taken to complete the verification.
\item Should not place significant demands on the cognitive abilities of the user.
\item Does not rely on culturally dependant knowledge. %Dubious 
\end{itemize}


